1. Technical Field
This invention relates generally to heat treat fixtures and methods for supporting thin sheet metal work pieces during heat treatment in liquid heating and cooling media to achieve hardening of the work pieces.
2. Related Art
The heat treatment of thin sheet-like structures such as metal gasket layers and the like is generally avoided in favor of the usage of pre-hardened materials such as 301 full hard stainless steel (FHSS) in the case of metal gasket layers. 301 FHSS starting material is roll hardened and possesses the desired end hardness and strength needed for metal gasket applications, which typically include one or more active layers formed with bead embossments that project out of the plane of the gasket layer and serve when compressed to provide a resilient seal between adjoining clamped structures such as a head and block of an engine. The bead embossments are typically stamped in an initially flat sheet of 301 FHSS via a stamping operation and, as such, the selection of material for the active layers must be sufficiently ductile to allow for such formation of the beads, yet sufficiently hard and strong in use to withstand considerable loading and deformation without cracking or yielding plastically under load.
It is generally accepted that the approach of heat treat hardening such thin sheet-like work pieces fabricated of less expensive heat treat hardenable materials, while attractive from a cost standpoint, is impractical at best, since such sheet-like structures having a considerably large surface area together with an extremely thin cross section (for metal gasket layers, typically on the order of about 0.01 inches) and are, by their nature, inherently unstable in a heat treat environment and would have a tendency to warp beyond levels acceptable in metal gasket applications when exposed to the extreme and rapid changes in temperature required to achieve heat treat hardening of the material.
Known approaches to controlling the warpage have involved constraining the thin sheets between two plates to prevent all movement of the sheet both against movement within its plane and out of its plane. Such approaches are not known to have been successful at preventing warpage and would likely worsen the condition by setting up non-uniform heating and cooling rates across the surface of the work piece which would contribute further to resultant warpage.
U.S. Pat. No. 5,310,196 discloses provision of a heat treat gasket layer, but the disclosure is silent as to the particulars of the heat treat process, including any fixturing of the parts, to achieve hardening while preserving the dimensional stability of the work pieces.
Accordingly, there is a need in the industry for a heat treat process and fixture apparatus capable of achieving effective, practical heat treat hardening of thin sheet work pieces such as metal gasket layers while preventing warpage of such work pieces that is prevalent using known conventional heat treatment and fixturing techniques.
According to one aspect of the invention, a fixture apparatus is provided for supporting at least one thin, planar sheet metal work piece fabricated of heat treat hardenable alloy during a heat treat cycle wherein the work piece is immersed in a first bath of liquid heating medium for heating the work piece to an elevated temperature followed by immersion in at least one bath of liquid cooling medium to quickly lower the temperature of the work piece to effect heat treat hardening of the work piece. The fixture apparatus comprises at least a pair of rigid support walls having oppositely disposed inner support surfaces that are substantially planar to define an envelope therebetween for the accommodation of at least one of such work pieces between the walls. The walls are coupled in such manner as to engage opposite sides of the work piece with the inner support surfaces in such manner as to support the work piece against movement out of its plane while permitting unrestricted movement of the work piece within its plane during the heat treat cycle. The walls are substantially porous to permit the free flow of the liquid heating and cooling medium through the walls for intimate contact with the work piece to achieve rapid uniform heating and cooling of the work piece, with the porosity being generally uniform across the support surfaces of the walls.
According to another aspect of the invention, a method is provided for heat treat hardening thin planar sheet metal work pieces fabricated of heat treat hardenable metal. The method comprises disposing the work piece in a fixture between opposing porous support walls thereof in such manner as to support the work piece against movement out of its plane while permitting the work piece to move within its plane. The fixture and work piece are immersed in a first bath of liquid heating medium which is permitted to flow through the porous walls of the fixture and intimately contact and uniformly heat the work piece to an elevated temperature. The fixture and work piece are then removed from the first bath and immersed in a second bath of liquid cooling medium which is permitted to flow through the porous walls of the fixture and intimately contact and uniformly cool the work piece to effect heat treat hardening of the work piece. Within the fixture, the work piece is substantially free to expand and contract within its plane during heating and cooling while being supported by the porous walls against movement out of its plane to prevent warpage of the work piece.
The invention has the advantage of providing a simple, effective means of heat treat hardening thin sheet-like work pieces such as metal gasket layers while avoiding the warpage problems associated with known conventional heat treatment and fixturing techniques.
The subject fixture and method enables less expensive materials to be used in thin metal layer applications, and particularly metal gasket layers. The use of such heat treat hardenable materials further reduces manufacturing costs by simplifying the formation of the usual bead embossments. Such materials are initially soft and readily deformable, requiring significantly lower stamping loads needed to form the embossments as compared to the force required to form such embossments in 301 FHSS starting material. The soft starting material further allows for greater design flexibility in the formation of the bead embossments, as there is not the concern for cracking the material.
The heat treat cycle has the added benefit of relieving any undesirable stresses that may have built up in the formation of the bead embossments that could contribute to early fatigue and failure of the gasket layer sometimes associated with full hard gasket layers.
The subject invention has the further advantage of enabling such work pieces to be austempered without warpage to achieve a desirable bainitic microstructure.